How does the lithium-ion battery work?

During the charge of the lithium-ion battery, reduction takes place at the negative electrode. Thereby, the negative electrode is forced to accommodate electrons from the positive electrode, which flows through the external current circuit. Simultaneously, the negative electrode inserts Li+-ions, which are extracted at the positive electrode side into the electrolyte and migrate and diffuse through the bulk electrolyte to the negative electrode side, to ensure the charge balance. As a result, the positive electrode active material is oxidized. In case of the discharge process, the redox reactions are inverted. During the discharge process, the negative electrode acts as anode and the positive electrode as the cathode. The electrochemical role of both electrodes’ changes between anode and cathode, depending on the direction of the current flow through the cell. However, throughout the literature, the positive electrode is named as the cathode and the negative electrode as the anode. The anode active material commonly used is graphite and the cathode active material is commonly a lithiated transition metal oxide, both being capable of Li+-ion insertion/de-insertion. As can be seen in the overall reaction of the lithium-ion battery , the active Li+-ions are shuttled between two insertion host electrodes in a reversible reaction during charge and discharge of the battery according to the so called “rocking chair principle”. Both electrodes are separated by a separator, which is soaked with a lithium salt-containing mixture of liquid organic solvents, that is the electrolyte, to ensure the fast transfer of Li+-ions within the cell.